Flouropolymers are melt-processable resins that are formed into polymer structures by many different processes, such as extrusion, injection molding, fiber spinning, extrusion blow molding and blown film. They are also used as polymer processing aids due to their low surface energies and phase behaviors.
The performance of polymeric resins during processing, especially their ease of processing, stability, and reliability, is essentially dominated by their viscoelastic properties. In particular, the polymer should exhibit strain hardening, shear thinning behavior, and a good balance between melt-strength and drawdown ratio. In addition, the polymer must retain good solid-state properties.
Fluoropolymers, typically made by an emulsion process, show moderate shear thinning behavior, and display poor melt strength for low molecular weight resins. They are generally linear and do not exhibit strain hardening upon an elongational deformation at small rates. As such, they are not very useful in applications such as blown films, extrusion blow molding, thermoforming and rigid foams.
Increasing the fluoropolymer molecular weight increases the melt strength, but often decreases the drawdown ratio. It is therefore difficult to obtain a balance between the parameters of melt strength and drawdown ratio without chemically changing the structure of the polymer. Crosslinked polymers may increase the melt strength, but are limited by the fact that they are not easily processable and often contain a high amount of gels.
One approach to achieve such a balance of properties is to introduce long chain branches onto the main backbone of the polymer. This allows for a wide range of architectures and hence wide ranges of melt rheological properties. There are various known ways to introduce long chain branching (LCB) onto the polymer backbone.
In olefin polymerization, catalysts are used to create controlled long chain branched polyethylene by copolymerization of ethylene with higher alpha-olefins, as described in WO 9612744 and Macromolecules (2003), 36(24), 9014-9019.
For polycondensation polymers, functional monomers are used to create LCB, as described in WO 2001066617 or branched diacid chains as described in Polymer Preprints (ACS Polymer Chemistry) (2002), 43(2), 472-473.
It has been shown that a gel-free highly branched polymer is difficult to achieve via emulsion polymerization. Journal of Polymer Science, Part A: Polymer Chemistry (1997), 35(5), 827-858.
Long chain branching has been achieved in polystyrene through the use of multifunctional initiators such as Luperox JWEB (Kasehagen et al., Society of Plastics Engineering, 2002 proceedings).
Fluoromonomers are very sensitive to hydrogen abstraction and conventional approaches to branching in other monomer systems, such as described above, cannot necessarily be used.
Several methods have been used to improve the long chain branching in flouropolymers. Macromolecular Symposia (2004), 206 (Polymer Reaction Engineering V), 347-360, discloses the use of reversible chain transfer based on iodine for the formation of long branches in a polymer. Branching can be induced by using bifunctional molecules that are able to link two different polymer chains to each other during polymerization. This is a 2 step process where the telomers have to be prepared separately. The present invention achieves the desired results without the use of such telomers.
Macromolecules (2000), 33(5), 1656-1663 discloses fluoropolymers in which trifunctional long-chain branches are originated by the transfer-to-polymer mechanism. In the present invention branching is achieved without the use of fluorinated diolefins.
U.S. Pat. No. 5,612,419 and U.S. Pat. No. 5,585,449 disclose a 2-step process using a bis-olefin to prepare fluorinated thermoplastic elastomers.
Branching through the use of low levels of radiation is disclosed in U.S. patent application Ser. No. 11/157,225
There is a need for a branched fluoropolymer that exhibits a low onset of shear thinning, and a good balance between melt-strength and drawdown ratio, while retaining good solid-state properties.
Surprisingly it has been found that fluoropolymers having long chain branching with the properties listed above can be achieved through the use of a certain persulfate initiators at very high temperatures. The branching can be optimized by using a co-initiator. One added advantage of this method is that the branched fluoropolymers contain little or no gels. These new materials would find application in areas where good melt rheology properties are required, such as blown films, fiber spinning, extrusion blow molding, thermoforming and rigid foams.